Another half-formed feature from indie cinema’s most prolific doodler, “Happy Christmas” demonstrates that Joe Swanberg makes better babies than movies. The evidence is Swanberg’s 2-year-old son Jude, who handily upstages anything co-stars Melanie Lynskey and Anna Kendrick can come up with to generate interest onscreen, to the extent that one can imagine a greater audience for 80 minutes of “baby Jude does the darnedest things” YouTube videos than might turn out for this sweetly insipid, mostly improvised dramedy, acquired by Magnolia and Paramount together prior to its Sundance premiere.

As it happens, Jude is also a more natural improviser than Swanberg’s professional cast, who presumably signed on for the experience of co-creating a film from within the confines of a broadly defined concept. Swanberg and Lynskey play young Chicago couple Jeff and Kelly, who invite Jeff’s younger sister Jenny (Kendrick) to crash in their tiki-themed basement for the holidays, falsely assuming that having an extra set of hands around will make it possible for Kelly to focus on her writing. Instead, Jenny proves to be a handful — another immature parental burden in a household where the grownups have enough to worry about with their biological child, Jude, demanding so much attention.

On her first night in town, Jenny sneaks out of dishwashing duties in order to go drinking with her buddy Carson (Lena Dunham, whose extemporaneous comedy instincts outstrip all her co-stars), which seems harmless enough until she blacks out at the party. The next morning, Jenny blows her babysitting duties, forcing Jude’s parents to call in a last-minute favor from their friend Kevin (Mark Webber, who cast his own 2-year-old son in “The End of Love”). Where a normal person might figure it’s a bad idea to seduce a family friend, Jenny gives Kevin her number, then drops by his apartment for some free pot and strings-attached sex.

From Jenny’s perspective, none of her behavior demands much of an apology. She’s simply being young and fun — which, despite Swanberg’s older-and-wiser outlook in this film, mirrors the plot’s unstructured, go-with-the-flow sensibility. Not that Jenny is an entirely unwelcome presence. When Kelly can’t figure out what to write, it is Jenny who suggests they bang out an erotic novel to score some quick money, an exercise that reawakens the exhausted mama’s creative spark (despite everything the agonizingly unfunny brainstorming scenes suggest to the contrary).

When partnered with the right collaborators, Swanberg’s approach can deliver flashes of genuine, unrehearsed brilliance — too many of which involve his son Jude’s adorable first stabs at speech this time around. Unfortunately, when the actors aren’t capable of surprising the audience along the way, the results feel slight and insubstantial. Here, Kendrick comes across especially ill-suited to the improv process, going through most of the pic with an insecure grimace plastered where a wider range of emotions might play in a properly scripted role.

“Happy Christmas” desperately needs some real jokes, rather than settling for the bemused chuckles that accompany its banal observations into human nature. Swanberg’s idea of a set piece involves a brief argument between the central couple, followed by an awkward non-confrontation as Jeff goes downstairs to sort things out with his sister, sitting in impotent silence for a couple minutes before exiting the scene, defeated, carrying her dirty dishes in hand.

It all builds to a climactic scene in which a charred pizza — rather than the sound of reindeer’s feet landing on the roof — awakens the entire family on Christmas Eve. Consider it Jenny’s gift to the family, though hardly a story worth repeating to Jude when he grows up, much less building a movie around.